In order to elucidate the role of the I antigen in disease states we must first isolate and fully characterize this material. I antigen will be isolated from human erythrocyte membrane using standard extraction procedures and/or by affinity chromatography. The chemical nature of this material (i.e. glycoprotein, glycolipid, etc.) will then be established and attempts will be made to determine the smallest active portion of this molecule by biochemical dissection using differential enzymatic digestion. That in fact this isolated material represents the I antigen in vivo as well as in vitro will be determined. The antigen's distribution on red cells using ferritin-labeling procedures will also be ascertained. Whereas there is no source of a standardized anti-I reagent we will attempt to develop one. Quantitative assay systems (quantitative immunoelectrophoresis and radioimmune assays) for the detection of either free I antigen or I antigen on cell surfaces or in determining the serum levels of anti-I will be developed using the purified I antigen and the anti-I reagent. Successful completion of the above proposal will lead to an activity study of the role of I antigen and its antibodies in different diseases.